1. Field of the Invention
This invention relates to the control of the luminous intensity of a cold cathode fluorescent lamp to be used for eliminating the electric charge or the like on the surface of a photoconductive photosensitive member (hereinafter referred to as a "photosensitive member") in an electrostatic reproducing apparatus (hereinafter referred to as a "reproducing apparatus") using the photosensitive member.
2. Description of the Prior Art
In a reproducing apparatus using a photosensitive member, reproduction is done by applying a uniform electrostatic charge to the surface of the photosensitive member, exposing it in accordance with a picture image so as to remove the electrostatic charge in accordance with the picture image and form an electrostatic latent image, forming a toner image on the surface of the photosensitive member by developing the electrostatic latent image, and thereafter transferring and fixing the toner image onto a transfer material such as transfer paper.
FIG. 1 illustrates portions of the reproducing apparatus to be applied with the present invention. Reference numeral 10 represents the photosensitive member (e.g. a drum); 101 is a charging means such as a corona discharger; 102 is optical exposing means for forming the electrostatic latent image; 103 is a developing means for forming the toner image; P is transfer paper that is placed on a paper feed tray; 104 is a paper feed roller for feeding the transfer paper P to the surface of the photosensitive member 10; 105 is a transfer/separation electrode for transferring the toner image to the transfer paper P and separating the transfer paper P with the toner image transferred to it from the surface of the photosensitive member 10; and 106 is a cleaner for removing residual toner from the surface of the photosensitive member 10 after the toner image is transferred.
To obtain a high quality picture it is extremely important to remove the residual electrostatic charge, and this is generally effected by exposing the surface of the photosensitive member 10 utilizing its photoelectric conductivity. (This procedure will be hereinafter referred to as "charge elimination.") Charge elimination is used not only to prepare an electrostatically uniform photosensitive member 10 prior to charging by the charging means but also to remove the electrostatic charge outside the region of the original on the surface of the photosensitive member 10 and for removing excess electrostatic charge other than the toner image before transfer.
In FIG. 1, reference numeral 11 represents charge eliminating means disposed upstream of the charging means 101 to remove the electrostatic charge on the surface of the photosensitive member 10 or make the fatigue of the photosensitive member uniform using light; and 12 represents partial exposing means that remove the electrostatic charge outside the region of the original when the optical system returns or during small-scale reproduction, and thus prevent the electrostatic charge from forming a dark frame around the picture image, from deteriorating the picture quality and from unnecessarily attaching to the surface of the photosensitive member 10 and being carried away and wasted. Reference numeral 13 represents exposing means before transfer that are interposed between the developing means 103 and the transfer/separation electrode 105, adjust the charge quantity of the electrostatic charge on the surface of the photosensitive member 10 and improve the transfer ratio of the toner image as well as separability of the transfer paper.
An incandescent lamp using the incandescent emission of a filament, a limit emitting diode (LED) or a fluorescent lamp has been employed as the light source for the abovementioned charge eliminating means 11, partial exposing means 12 and exposing means before exposure 13.
Among the abovementioned light sources, a plurality of incandescent lamps or LEDs must be arranged in order to illuminate a required area, so that the distribution of the luminous intensity becomes non-uniform and hence the charge elimination and optical fatigue of the photosensitive member are likely to be non-uniform. The incandescent lamp generates a lot of heat so that the photosensitive member is also likely to be degraded by the heat.
Since the fluorescent lamp is free of the abovementioned drawbacks, it is, in this sense, a suitable light source for charge elimination. However, since the vapor pressure of mercury sealed in the tube markedly varies with the temperature, the light emitting luminous intensity is significantly affected by the temperture inside the tube. FIG. 2 illustrates the relationship between them. The ordinate represents relative luminous intensity, which is plotted at 100% when the temperature of the tube wall is at 40.degree. C., and the abscissa represents the tube wall temperature, which is substantially proportional to the temperature inside the tube, and is used herein as the temperature. As is obvious from this diagram, the relative luminous intensity shows a change of about 60% within a temperature range of from 10.degree. C. to 40.degree. C.
The temperature inside the tube of the fluorescent lamp changes with the ambient temperature of the fluorescent lamp that is determined by the conditions inside the reproducing apparatus, the place of installation and the season, and by the temperature rise inside the tube due to the heat that is generated by the discharge current of the lamp itself, though the heat generation is much smaller than that of an incandescent lamp.
Various problems such as photographic fog, drop of the toner transfer efficiency, so-called "jamming" of the transfer paper and the like when the fluorescent lamp is used as the light source for the charge elimination occur especially frequently when the temperature inside the tube of the fluorescent lamp is low. The state changes depending upon the time it has been lit because of the heat generated by the discharge current.
A cold cathode type fluorescent lamp (hereinafter referred to as the "cold cathode lamp") is available as a suitable lamp that does not show the unstability of the luminous intensity of the fluorescent lamp. The lamp current and relative luminous intensity of this cold cathode lamp show a good linear relation. FIG. 4 is a diagram showing this relation between the relative luminous intensity and the lamp current in which the luminous intensity is plotted at 100 when the lamp current is 5 mA. This lamp current can be easily changed by changing the output of a transformer 25 on its secondary side or a resistor R shown in FIG. 3 described below.
The cold cathode lamp is a quick starting type, has a small volume of about 1/3 that of the ordinary fluorescent lamp, and is more economical because it does not need an auxiliary device for lighting. The cold cathode lamp and its associated circuit are shown in FIG. 3. In the drawing, reference numeral 20 represents the cold cathode lamp; 21 is the fluorescent tube of the cold cathode lamp; 22 and 22' are electrodes disposed at both ends of the fluorescent tube 21; and 23 and 23' are caps. Reference numeral 24 is a member which may be called as an "adjacent conductor" which is extended from one 22 of the electrodes along the outer wall of the fluorescent tube 21 (on the side of atmosphere) to close to the other electrode 22' but does not come into contact with it, in the example shown in FIG. 3. This is made of a conductive paint film. Reference numeral 25 represents a transformer for passing current through the cold cathode lamp 20 and symbol R represents a resistor interposed between the transformer 25 and the cold cathode lamp 20 to control the lamp current.
When an a.c. voltage of 300 to 700 V is applied across the electrodes 22 and 22', discharge occurs between the adjacent conductor 24 and the electrode 22' adjacent the former, this discharge functions as a trigger and discharge occurs instantaneously and successively between the electrodes 22 and 22', thereby turning the lamp on. The lamp current of the cold cathode lamp required for discharge after lighting is from 1 to 10 mA and is much smaller than the lamp current on the order of several hundreds of mA of the ordinary fluorescent lamp. Accordingly, heat generated in the lamp by the lamp current can be substantially neglected, and the temperature of the fluorescent tube will be substantially equal to the ambient temperature.
As described above, the cold cathode lamp has various advantages in comparison with an ordinary fluorescent lamp. Since the principle of light emission of the cold cathode lamp is the same as that of the ordinary fluorescent lamp, however, the luminous intensity of the emitted light of the cold cathode lamp depends upon the temperature in the same way as in the ordinary fluorescent lamp as illustrated in FIG. 2. Nonetheless, heat generated of the cold cathode lamp itself can substantially be neglected, and since the relative luminous intensity is substantially proportional to the lamp current, the luminous intensity of the fluorescent lamp can be easily controlled by controlling the lamp current.
On the other hand, in order to provide a copy having high picture quality and to avoid problems such as jamming, the quantity of light emitted to the photosensitive member from the charge eliminating means 11, partial exposing means 12 and exposing means before transfer 13 described with reference to FIG. 1 must be maintained within practical tolerances. However, there have not been made any proposals in the past to maintain successively the luminous intensity of the light source for charge elimination or the quantity of light emitted.